This invention relates to improved methods for producing urethane foam and the like and, more particularly, relates to improved methods for mixing liquid reactants for the production of foam.
One of the techniques for the production of urethane foam is to continuously mix reactive materials while continuously pouring them. This, of course, involves the need for providing a continuous flow of reactive mixture from a discharge nozzle at suitable volumes with proper mixing so that the deposited mixture reacts to provide an acceptable product.
In continuous molding of rigid foam, a reactive mixture is deposited on a moving conveyor at the input side of a tunnel mold and, as the reaction and foaming occur, the mixture expands and is defined in configuration by the cross-section of the tunnel mold and solidifies into a predetermined configuration. The quality of the resulting product is heavily dependent upon the quality of the mixing of the materials which are deposited for the reception by the mold.
In continuous molding of flexible foam, a reactive mixture is deposited on a moving conveyor at the input side of a U-shaped form, and, as the reaction and foaming occur, the mixture expands and is defined by the cross-section of the form.
There are, of course, other types of foam molding other than with a continuously moving conveyor. In almost all instances, proper mixing is essential to the formation of a good product.
As noted heretofore, urethane foam products can be produced by a number of techniques and can be either rigid or flexible. The present invention is principally concerned with a mixer for high volume, short time and intimately mixed throughputs where the chemical components are supplied to the mixer.
The raw materials used to produce foams typically consist of a di- or polyfunctional isocyanate, a polyfunctional hydroxyl material, a blowing agent, catalysts, cell size regulators, surfactants, and sometimes fillers or additives to obtain special properties. For the production of rigid foams, a relatively viscous hydroxyl compound is employed, and the objective is to obtain thorough mixing and dispersion of the various ingredients; the chemicals are such that about ninety percent of the cells in the foam product are closed. On the other hand, for flexible foams the polyester and polyether hydroxyl compounds are much less viscous, and the cells in the foam product are substantially all open. Obviously, viscosity of the chemical ingredients plays an important part in mixing and the likelihood of obtaining a thorough mixing action. The mixing head is supplied with a preblended mixture of hydroxyl, catalyst, coloring and acid which do not produce the foaming reaction. The other main mixture supplied to the mixer head is the isocyanate. Thus, the function of the mixing head is to blend the isocyanate and hydroxyl mixtures as quickly and thoroughly as possible to provide the reactive mixture. In this system, it is necessary to use the preblended mixture as soon after preblending as possible, since chemical reactions can occur.
In U.S. Pat. No. 3,482,822, a mixing head is illustrated which has some features in common with the present invention. In the U.S. Pat. No. 3,482,822, a preblended hydroxyl mixture was combined with an isocyanate, and mixing of the isocyanate and preblended mixture were accomplished by intermeshing pins on the barrel and a whipper. This was highly satisfactory for certain formulations but lacks a controlled and proper mixing for faster reaction formulations. In any event, this head did not meet the entire set of problems posed by foam formulations.
To mix compounds for flexible foam, heretofore it has been necessary to use a high pressure injection system for the ingredients in the order of 2000 psi. The ingredients are injected into a mixing chamber through an injection nozzle system to provide a finely atomized, high velocity stream for mixing and dispersion purposes. This system obviously has drawbacks. With the present invention, relatively low pressure mixing of compounds for flexible foam can be accomplished.
The mixing head can have outputs from 90 to 600 pounds/minute and typically traverses a moving conveyor at right angles while the liquid is laid down in a pattern of nearly parallel lines which merge before creaming. Upon mixing, the reaction begins immediately, and a foam rise can start at as little as two seconds after mixing (and even faster reactions are possible) and be completed within 1 to 2 minutes. Obviously, therefore, there is little time available for mixing. However, one of the most significant factors in producing good foam is proper mixing, particularly to handle dispersion of the vaporizing liquid and complex systems involving more than one reactant to obtain very highly cross-linked systems. A part of the difficulty in mixing the components for a urethane foam stock is the fact that some of the constituents are immiscible with one another, and others produce undesired reactions if not properly combined.